I had a few notes on technique and such. The most immediate is that your brush strokes need to follow her fur. Fur typically flows in a vertical direction on the neck, and your strokes appear to make them flow horizontally. This is good for brushed steel and other metals, as well as turned wood, but on fur and other fibrous surfaces the brush strokes need to follow the "knap" of the fur.

Another thing I noticed is that the shading that indicates the interior of the ear should continue further up the ear, so that it makes the ear look less "thick".

Finally, the edges of the mane where the hair curves around should be rougher that the sides of the mane. This makes it look like hair rather than a sheet of brushed plastic.

Since Critiques are over, I'll do it on the comment section :3Now,as a fan of Rarity, I'm gonna go ahead and say, this is great! :3 The way you drew her is wonderful, and great shading, and the eyes... Are just...Oh my gosh... STUNNING. But the mane is good,just needs more detail and a little more shading on that part. But otherwise, the portrait is really good! Worthy of a fav. <3

Beautiful! I like the way the color blends in her hair, and her proportions look great.

I have a suggestion: consider making the "brush strokes" follow the direction of her fur (if she had fur). For instance, on her neck, having the strokes run vertically instead of horizontally. It will give a natural look to her skin.